1. Technical Field
The present invention relates to a receiving apparatus for receiving a digitally modulated signal and particularly to a correcting technology for a received frequency.
2. Description of the Related art
A receiving apparatus for receiving a digitally modulated signal shown in FIG. 6. A digitally modulated signal is received by an antenna 11, and after the received signal is amplified by a high frequency amplifier 12, it is mixed with an output of a frequency synthesizer 14 at a frequency converter 13 and requency-converted to a predetermined intermediate frequency signal. After the converted intermediate frequency signal is amplified by an intermediate frequency amplifier 15 and passed through a band limiting filter 16, it is supplied to an orthogonal detector 17 for orthogonal detection and becomes a base band signal. The orthogonal detection is carried out in synchronization with a frequency of a carrier wave supplied from a frequency synthesizer 18.
The base band signal converted by the orthogonal detector 17 is supplied to a demodulator 19 and the demodulated data is error-corrected by an error-correcting circuit 20 and supplied to a voice decoder 21 to be subjected to a voice decoding process, the decoded voice signal is supplied to a voice output circuit (not shown) at a later stage.
Here, the frequency which is frequency-converted by the frequency converter 13 and the frequency which is detected by the orthogonal detector 17 is determined by the frequency supplied from the frequency convertor 13, but frequency correction needs to be carried out for the frequencies supplied from the frequency synthesizers 14, 18 to synchronize with the frequency of the received signal.
FIG. 2 is a block diagram showing an arrangement for carrying out the conventional frequency correction. The frequency correction is carried out based on a received signal symbol demodulated by the demodulator 19. That is, a frequency offset of the received signal symbol demodulated by the demodulator 19 is detected by a frequency offset detector 31 and the value of the detected frequency offset is averaged by being integrated by a digital integrator 32. The averaged value integrated by the integrator 32 is converted to an analog voltage by a digital/analog convertor 33 and the converted voltage is supplied to a control voltage input of a voltage controlled oscillator 34 to control the output frequency of the voltage controlled oscillator 34. An output of the voltage controlled oscillator 34 is supplied to the frequency synthesizers 14, 18 to control their output frequencies.
By the control in this circuit, the correction of the received frequency can be carried out. First of all, in regard to the digital modulated-signal to be received, if a digital information signal of an in-phase component is assumed to be I(t) while a digital information signal of an orthogonal component is assumed to be Q(t), a signal X(t) of the information component is defined by a complex number such as X(t)=I(t)+jQ(t). The digitally modulated signal can be expressed in the following.
s(t)=Re(x(t)xc2x7ej2xcfx80fct)xe2x80x83xe2x80x83[Equation 1]
Here, fc denotes a carrier wave frequency and the carrier wave is expressed by a complex sinusoidal wave c(t) =ej2xcfx80fct.
The digitally modulated received signal is, by being subjected to an orthogonal demodulation on the receiving apparatus side, extracted as a signal of the information component as a base band signal. The orthogonal demodulation will be explained hereinafter. The digitally modulated received signal is demodulated in the following way by being multiplied by a received carrier wave with a reverse phase rotation relative to the carrier wave.                                                                         F                ⁢                                  (                                                            s                      ⁡                                              (                        t                        )                                                              ·                                          ⅇ                                                                        π                          ⁢                                                      xe2x80x83                                                    ⁢                          f                                                -                                                  2                          ⁢                                                      xe2x80x83                                                    ⁢                          π                          ⁢                                                      xe2x80x83                                                    ⁢                          fct                                                                                                      )                                            =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                  F                  [                                      (                                                                                            x                          ⁡                                                      (                            t                            )                                                                          ·                                                  ⅇ                                                      j2π                            ⁢                                                          xe2x80x83                                                        ⁢                            fct                                                                                              +                                                                                                                                                                                                    xe2x80x83                                    ⁢                                      x                    *                                                                  (                        t                        )                                            ·                                              ⅇ                                                                              -                            j2π                                                    ⁢                                                      xe2x80x83                                                    ⁢                          fct                                                                                                      )                                ·                                  ⅇ                                                            -                      j2π                                        ⁢                                          xe2x80x83                                        ⁢                    fct                                                              ]                                                                          =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                  F                  [                                                            x                      ⁡                                              (                        t                        )                                                              +                                          x                      *                                                                        (                          t                          )                                                ·                                                  ⅇ                                                                                    -                              j                                                        ⁢                                                          xe2x80x83                                                        ⁢                            2                            ⁢                            π                            ⁢                                                          xe2x80x83                                                        ⁢                            fct                                                                                                                                                                                                                      =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                  (                                                            X                      ⁡                                              (                        f                        )                                                              +                                          x                      *                                              (                                                                              -                            f                                                    -                                                      2                            ⁢                            fc                                                                          )                                                                              )                                                                                        [Equation  2]            
Here, X(f) denotes a frequency of x(t) and x*(t) and X*(f) express complex conjugates of x(t) and X(f). By taking out only a low-band signal from the demodulated received signal by a low-pass filter, the base band signal can be obtained.
When there exists a frequency offset f between the carrier frequency used for the digital modulation and the frequency of the complex sinusoidal wave used for the orthogonal demodulation, the reception signal converted by the orthogonal demodulation is expressed in the following equation and the base band signal obtained after the low-band signal is extracted by the low-pass filter is, as shown in FIG. 3, observed as time phase rotation of an angular velocity xcfx89=2f on an IQ plane.                                                                         F                ⁡                                  (                                                            s                      ⁡                                              (                        t                        )                                                              ·                                          ⅇ                                                                        -                          j                                                ⁢                                                  xe2x80x83                                                ⁢                        2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                                                  (                                                      fc                            +                                                          Δ                              ⁢                                                              xe2x80x83                                                            ⁢                              f                                                                                )                                                ⁢                        t                                                                              )                                            =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                  F                  [                                      (                                                                                            x                          ⁡                                                      (                            t                            )                                                                          ·                                                  ⅇ                                                      j                            ⁢                                                          xe2x80x83                                                        ⁢                            2                            ⁢                            π                            ⁢                                                          xe2x80x83                                                        ⁢                            fct                                                                                              +                                              x                        *                                                                              (                            t                            )                                                    ·                                                                                                                                                                                                            xe2x80x83                            ⁢                                                ⅇ                                                                                    -                        j2π                                            ⁢                                              xe2x80x83                                            ⁢                                              (                                                  fc                          +                                                      Δ                            ⁢                                                          xe2x80x83                                                        ⁢                            f                                                                          )                                            ⁢                      t                                        )                                                  ·                                  ⅇ                                                            -                      j2π                                        ⁢                                          xe2x80x83                                        ⁢                                          (                                              fc                        +                                                  Δ                          ⁢                                                      xe2x80x83                                                    ⁢                          f                                                                    )                                        ⁢                    t                                                              ]                                                                          =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                  F                  [                                      (                                                                                            x                          ⁡                                                      (                            t                            )                                                                          ·                                                  ⅇ                                                                                    -                              j2π                                                        ⁢                                                          xe2x80x83                                                        ⁢                            Δ                            ⁢                                                          xe2x80x83                                                        ⁢                            ft                                                                                              +                                              x                        *                                                                                                                                                                                    xe2x80x83                            ⁢                                                (                  t                  )                                ⁢                                  ⅇ                                                            -                      j2π                                        ⁢                                          xe2x80x83                                        ⁢                    2                    ⁢                                          (                                              fc                        +                                                  Δ                          ⁢                                                      xe2x80x83                                                    ⁢                          f                                                                    )                                        ⁢                    t                                                              )                                                                          =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                  (                                                            X                      ⁡                                              (                                                  f                          -                                                      Δ                            ⁢                                                          xe2x80x83                                                        ⁢                            f                                                                          )                                                              ⁢                    X                    *                                          (                                                                        -                          f                                                -                                                  2                          ⁢                                                      (                                                                                          f                                c                                                            +                                                              Δ                                ⁢                                                                  xe2x80x83                                                                ⁢                                f                                                                                      )                                                                                              )                                                        )                                                                                        [Equation  3]            
Since this causes a serious influence on a code judgement error of the received signal, it is necessary to measure transmission and received frequency offsets as well as correcting a received carrier wave frequency on the receiving apparatus side.
Next, a process for estimating the frequency offset will be explained with reference to the system shown in FIG. 2. When a received signal symbol rt outputted by the demodulator 19 of FIG. 1 and a transmission signal symbol corresponding to a previously demodulated received signal symbol rt-n are identical, by carrying out an arctan operation to a value obtained by complex conjugate multiplication between rt and rt-n, a phase rotation angle xcex94xcex8 can be detected. By obtaining the phase rotation angle xcex94xcex8 which rotates on the IQ plane at some constant interval xcex94t, the frequency offset xcex94f can be obtained by the following.       Δ    ⁢          xe2x80x83        ⁢    f    =            Δ      ⁢              xe2x80x83            ⁢      θ              2      ⁢              xe2x80x83            ⁢              π        ·        Δ            ⁢              xe2x80x83            ⁢      t      
In the conventional process for estimating a frequency offset, the following problems arise. When a phase rotation angle between symbols is detected, an arctan operation is necessary to obtain a value which is obtained by complex conjugate multiplication of the two symbols, but since the amount of calculation of the arctan operation is comparatively large, when a real time process is needed, it can happen that the arctan operation process does not end within a required time. In order to avoid this, the following approximation has been carried out. Here, amplitude rtk is denoted as At, a phase angle as xcex8t, an in-phase signal as it and an orthogonal signal as qt.                                                                                           r                  t                  xe2x80x3                                ⁢                                  r                                      t                    -                    n                                    *                                            =                              xe2x80x83                            ⁢                                                A                  t                                ·                                  ⅇ                                      j                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      t                                                                      ·                                  A                                      t                    -                    n                                                  ·                                  ⅇ                                                            -                      j                                        ⁢                                          xe2x80x83                                        ⁢                                          θ                                              t                        -                        n                                                                                                                                                                    =                              xe2x80x83                            ⁢                                                A                  t                                ·                                  A                                      t                    -                    n                                                  ·                                  ⅇ                                      j                    ⁢                                          xe2x80x83                                        ⁢                                          (                                                                        θ                          t                                                -                                                  θ                                                      t                            -                            n                                                                                              )                                                                                                                                              =                              xe2x80x83                            ⁢                                                A                  t                                ·                                  A                                      t                    -                    n                                                  ·                                  ⅇ                                      jΔ                    ⁢                                          xe2x80x83                                        ⁢                    θ                                                                                                                          =                              xe2x80x83                            ⁢                                                A                  t                                ·                                  A                                      t                    -                    n                                                  ·                                  (                                                            cos                      ⁢                                              xe2x80x83                                            ⁢                      Δ                      ⁢                                              xe2x80x83                                            ⁢                      θ                                        +                                          j                      ⁢                                              xe2x80x83                                            ⁢                      s                      ⁢                                              xe2x80x83                                            ⁢                      in                      ⁢                                              xe2x80x83                                            ⁢                      Δ                      ⁢                                              xe2x80x83                                            ⁢                      θ                                                        )                                                                                                        =                              xe2x80x83                            ⁢                                                (                                                            i                      t                                        +                                          jq                      t                                                        )                                ·                                  (                                                            i                                              t                        -                        n                                                              -                                          jq                                              t                        -                        n                                                                              )                                                                                                        =                              xe2x80x83                            ⁢                                                (                                                                                    i                        t                                            ·                                              i                                                  t                          -                          n                                                                                      +                                                                  q                        t                                            ·                                              q                                                  t                          -                          n                                                                                                      )                                +                                  j                  ⁡                                      (                                                                                            q                          t                                                ·                                                  i                                                      t                            -                            n                                                                                              -                                                                        i                          t                                                ·                                                  q                                                      t                            -                            n                                                                                                                )                                                                                                          [Equation  5]            
By utilizing the approximation that xcex94xcex8=sin xcex94xcex8 is true when xcex94xcex8 is near to zero, the phase rotation angle xcex94xcex8 can be expressed in the following formula.
xcex94xcex8=qtxc2x7it-nxe2x88x92itxc2x7qt-nxe2x80x83xe2x80x83[Equation 6]
When this approximation is used, however, the phase rotation angle of the frequency offset which can be detected is limited to within a range of xc2x1xcfx80/2. When the phase rotation angle exceeds the range of xc2x1xcfx80/2, the frequency offset is erroneously detected. Also, according to the equation 5, since a presumed value of the phase rotation angle changes depending on a strength of the received signal, there is a change in a presumed value of the frequency offset, thereby incurring variations in converging time and correction errors. Also, since the interval between the received signal symbols used for detecting the conventional frequency offset is constant, there have been problems that when the interval between the symbols is narrow, although a frequency pulling-in range becomes wide, the frequency correction error becomes large after the pulling-in, while when the interval between the symbols is wide, although the frequency correction error becomes small after the pulling-in, the frequency pulling-in range becomes narrow.
An object of the present invention is to make it possible to favorably carry out correction of the received frequency when receiving a digitally modulated signal.
The present invention comprises phase judging means for judging a phase of a received signal symbol by the predetermined number of quantization, phase difference detecting means for detecting a phase difference between phase-judged samples judged by the phase judging means and averaging means for averaging a detecting output of the phase difference detecting means, to thereby control an output frequency of a frequency synthesizer based on an output of the averaging means.
According to such an arrangement, the phase judging means digitally judges the phase by the predetermined number of quantization, and a difference between the judged phase value and a phase value immediately before is detected by the phase difference detecting means and the detected phase difference is predeterminately averaged by the averaging means and the output of the frequency synthesizer is controlled based on the averaged frequency error and a frequency subjected to a received process is corrected.